# Biexcitons or drop embryos in CdS, CdSe

## Abstract

The transition probability P(K, n) of the reaction {Mathematical expression} has been computed in direct-gap semiconductors for several final states of the exciton (1 s or 2 p)vs. the biexciton kinetic energy. The probability P(K, n) is found to decrease strongly with K, whereas it is usually assumed that P(K, n) is constant in the analysis of many experiments (luminescence of biexcitons, induced formation of biexcitons, gain measurement of the biexciton light) on the basis of a simple calculation. The line shape theory used in the preceding experiments is, therefore, incorrect, especially in semiconductors with exciton of small binding energy (such as CdSe or CdS). The previously proposed interpretation of the so-called M s band in terms of biexcitons in CdS or CdSe is very questionable on the basis of this calculation and recent picosecond-time-resolved luminescence experiments which cannot be analysed in the biexciton model. We support the interpretation of this M s band in terms of bound polyexcitons and drop embryo luminescence. © 1993 Società Italiana di Fisica.